1. Field of the Invention
The present invention relates to methods of exploring for oil or gas deposits. Particularly, the present invention relates to methods for predicting the likelihood of finding oil or gas deposits at unexplored depths beneath selected locations by utilizing earth surface temperatures. More particularly, the present invention relates to methods for predicting the likelihood that oil or gas deposits are present beneath selected locations by comparing the earth surface temperature at a selected location with earth surface temperatures in the surrounding geographic area. Anomalously low earth surface temperatures at a location indicate that oil or gas deposits are more likely to exist beneath that location than under locations with higher temperatures.
2. Description of Pertinent Art
In U.S. Pat. No. 4,476,716, incorporated by reference into this application in its entirety, I disclosed methods for predicting the likelihood that oil or gas deposits are present at unexplored depths below a selected location. These methods were based upon the discovery that temperatures at a given subsurface depth at a location above an oil or gas deposit tend to be lower than temperatures at the same depth at other locations in the same geographic area where oil and gas deposits do not exist. A principal method disclosed in U.S. Pat. No. 4,476,716 for predicting the likelihood that oil or gas deposits exist below a selected location comprises: determining an actual temperature (T.sub.a) at a known depth beneath the location; determining an average temperature (T.sub.r) for the known depth in the geographic area surrounding the location; and determining the temperature difference (.DELTA.T=Ta-Tr) between the temperature at the location and the average temperature, and predicting that oil or gas deposits are likely to be present below the known depth at the location for negative values of .DELTA.T, or predicting that oil or gas deposits are unlikely to be present below said known depth at said location for positive values of .DELTA.T.
In U.S. Pat. No. 4,476,716 it was observed that the surface of the earth, and very near-surface are affected by seasonal fluctuations in atmospheric and benthic temperatures, and at the near-surface depth to which seasonal temperature changes reach, the temperature of the earth is in many cases constant at about the value of the yearly average surface temperature. From this near-surface depth limit, the temperature of the earth begins its steady increase proportional to depth. Also, it was observed that temperatures and geothermal gradients for the same depth intervals vary significantly from well to well, even within small geographic areas. And finally, it was observed that temperatures at a first location above an oil or gas deposit tend to be lower than temperatures at corresponding depths at other locations, in the same geographic area with the first location, where oil or gas deposits do not exist.
Critical to the effectiveness of the methods in U.S. Pat. No. 4,476,716, is acquiring accurate subsurface temperature data such that temperature at a known depth below a selected location and average temperature for that depth in the geographic area surrounding the location are accurately known. As exemplified in claims 11 and 12 of U.S. Pat. No. 4,476,716, earth surface temperatures were believed to be affected by the presence or absence of oil or gas deposits below locations to such an extent that average annual atmospheric temperatures above such locations would be affected by a measurable amount. The magnitude of the difference in temperature between locations where oil or gas deposits do and do not occur increases from the surface to the depth just above where the oil or gas deposit is located. Thus, at the surface the temperature above an oil or gas deposit will be only slightly (but measurably) less than the average surface temperature for the geographic area. However, at a depth just above the oil or gas deposit, the temperature is much lower, often in the range of 40.degree. F. to 60.degree. F. lower, than the average temperature for such depth. The magnitude of the observable decrease in temperature above an oil or gas deposit is directly proportional to the size and thickness of the oil or gas deposit.
As exemplified in claims 11, 12 and 13 of U.S. Pat. No. 4,476,716, temperatures at particular locations, both in air and in water, are believed to be affected by the presence or absence of oil or gas deposits below the locations, and that sufficiently accurate temperature data (such as average annual atmospheric temperature data taken over many years) for particular locations and their surrounding geographic areas can be effectively used in the methods of U.S. Pat. No. 4,476,716 to predict the likelihood of oil or gas deposits existing below a particular location. Such average annual atmospheric (or benthic) temperature data is available for only a few locations, and is unavailable for the great majority of locations which are attractive sites for exploring for oil or gas. Collecting such average annual temperature data for even one location and its surrounding geographic area is prohibitively expensive. Collecting such average annual temperature data requires installation and monitoring of permanent temperature measuring installations at a number of places, including locations of interest, in the geographic area of interest for at least several years.
Surface temperature differences of significance in the search for oil or gas deposits are small. A large temperature deviation from average, in the range of 40.degree. to 60.degree. F., at the depth of an oil or gas deposit, will, when projected to the earth's surface, be reduced to a temperature deviation in the range of about 1/2.degree. F. to 5.degree. F., depending upon specific conditions at each location and geographic area measured.
Earth surface temperatures are affected not only by seasonal changes in temperature, but also by more immediate factors, such as diurnal effects, fluctuations in incident radiation (sunlight), ambient temperature, soil moisture, atmospheric humidity, wind desiccation, cloud cover, atmospheric clarity, precipitation, soil type, reflectance/emissivity of the surface, vegetative cover, elevation above/below sea level, local topography and surface features, etc. These factors tend to create temperature differences which mask any temperature differences between a location and its surrounding geographic area arising from the presence or absence of oil or gas deposits below the location. While time averages, such as average annual atmospheric temperatures, tend to eliminate the masking effect of these factors such that earth surface temperature effects due to the presence or absence of oil or gas deposits can be observed, obtaining such time average temperature data is not economically or functionally practical.
Consequently, a practical method for acquiring temperature data at the earth's surface and near-surface, which data is sufficiently accurate to show effects of the presence or absence of oil or gas deposits beneath the surface, are desirable for expanding the capacity for exploring for oil and gas reserves.